the
blackness will be uniform; but if, as often happens, its constitution
is not very regular, the most porous parts absorb the greater quantity
of oil, and varied effects of colouring are produced, which furnish to
the artist the desirable opposition of tints.
This
operation can be easily explained. The oil being composed of the three
elements—carbon, hydrogen, and oxygen, it follows that if the hydrogen
and oxygen are removed, carbon remains. It is precisely this removal
that sulphuric acid effects. Penetrating in the track of the oil into
the pores of the stone, it determines the union and elimination of the
hydrogen and oxygen, and leaves carbon diffused throughout the stone in
a state of excessive division. It is these minute particles of carbon
that impart a definite colour to the stone.
So
far as regards the colouring, this process furnishes excellent results;
but when it is asked, Should the prepared stone be considered equal to
those coloured by nature? we must answer in the negative,
notwithstanding the opinion generally held.
No
doubt, in ordinary conditions, the fixity of carbon is absolute, but in
this case we must remember that it is in a state of the most extreme
division ; and seeing that the natural porosity of the stone is
increased by the sulphuric acid, and that porosity is highly favourable
to the combination of
